40 Dr Gaskell, The Origin of Vertebrates. [Dec. 2, 



is a piece of the dorsal ectoderm. In the Arthropod it is equally 

 certain that the nervous system separates from the ectoderm in 

 the ventral middle line [of the Arthropod], and is therefore a 

 piece of the ventral ectoderm. Nevertheless Dr Gaskell held 

 both that the nervous system of the Vertebrate was homologous 

 with that of the Arthropod, and that it was the dorsal surface 

 of the Arthropod which corresponded with the dorsal surface of 

 the Vertebrate. Dr Gaskell had made no attempt to reconcile 

 the different modes of development in the two cases. 



Extraordinary changes of structure and function were postu- 

 lated by Dr Gaskell. These were supposed to have occurred in 

 evolution, in the course of lineal descent, by the process of varia- 

 tion. If anyone would try to conceive these variations taking 

 place before his eyes, he would realize the difficulties involved. 

 For example if it was alleged that a specimen of Limulus had 

 been found which had developed a mouth in its operculum, or 

 a perforation from the space occupied by the branchiae leading 

 into the intestine, or feeding or digesting by means of that space, 

 or with its nervous system enveloping its alimentary canal, no 

 one would credit the observation without overwhelming testimony. 

 But Dr Gaskell assumed that all these things and many others 

 no less astonishing had occurred as variations. No evidence had 

 been offered that any such variations or any approach to them 

 had ever occurred, or even that they were possible. 



In the absence of such evidence there was no reason why 

 Dr Gaskell's suggestion should be seriously considered at all. 



Dr Gaskell had mentioned Pterichthys, Coccosteus, and other 

 fossil Fishes, hinting that he regarded them as possible links 

 between Arthropods and Vertebrates : had Dr Gaskell any reason 

 for supposing that these animals were anything but Fishes in the 

 strictest sense ? 



Mr Shipley said he thought that in attempting to derive 

 one group of animals from another, certain principles were usually 

 observed, (i) one of these was that in assuming variations to arise 

 in an ancestor the amount of change assumed and its direction 

 should be checked by the kind of variation which is already 

 known to exist in the various members of the group to which 

 the ancestor belongs, but although no one could deny that the 

 members of the Arthropoda varied in almost every possible 

 direction, none of them varied along the lines laid down by 

 Dr Gaskell; (ii) the amount of variation assumed should be 

 limited : there was no fact known amongst animals which led 

 us to believe that they ever lost and redeveloped such funda- 

 mental organs as the entire alimentary canal, the renal organs 

 and the reproductive organs, yet this was assumed in Dr Gaskell's 

 hypothesis; (iii) changes of function can arise when the new 





